What about putting in U-shaped rail the length that the bike will be in the tailgate with a condor wheel chock up front. Firstly that will help hold the bike up while you strap it but the u-rail would take the load off the tailgate. It would be like my motorcycle trailer which is a thin single rail unit but with just the rail and wheel chock in the back of the truck. I'd think getting that put together wouldn't cost much money. There is even an inexpensive wheel chock available at Harbor Freight. Here is my trailer for a visual:
https://www.stingertrailer.com/produ...lding-trailer/
Other option is do you get the back wheel off the tailgate if you put the bike in the back diagonally?
Just a couple of crazy ideas!
Joe